I am certain there are other comics being used in this manner, but that really was the whole "point." Sinfest is a mishmash of these things, with its own humour. In many ways, it's like a DJ/producer taking a great song, then remixing it so it's more or less completely new, awesome in a different way, yet deep down here and there are the elements of the old. I happen to like remixes, and consider this to be an art.

I'm most familiar with Calvin and Hobbes, and can think of a handful of strips that are very similar to Watterson's. However, at the end of the day, the overall humour from Sinfest is of its own and by no means borrowed or ripped off.